Protein kinases (PKs), also called protein phosphokinases, are a sort of enzymes that catalyze the protein phosphorylation reaction. The protein kinases exert their physiological functions, including cell growth, survival and differentiation, organ formation and morphological change, neovascularization, tissue repair and regeneration, by catalyzing the phosphorylation of a protein. In addition to normal physiological functions, many protein kinases play an important role in human diseases (such as cancer). Cancerogenic protein kinases, i.e., a subgroup of protein kinases, when dysregulated, may cause tumor formation and growth, and further cause tumor metastasis and progression. To date, the cancerogenic protein kinases are one of the most important targets for treating cancers.
The protein kinases can be classified into receptor type and non-receptor type. A subfamily of the non-receptor type of tyrosine kinases (PTKs) comprises Janus kinase (JAK). As for the non-receptor type of tyrosine kinases, reference can be made in detail to, e.g., Bolen J B., Nonreceptor tyrosine protein kinases, Oncogene, 1993, 8(8): 2025-31.
Janus kinase (JAK) is a non-receptor type of tyrosine kinases (PTKs), which resides in cells and transduces cytokine stimulation signal via JAK-STAT pathway. By JAK-STAT pathway, a chemical signal outside the cell is transduced into a gene promoter on endonuclear DNA through cell membrane, and finally affects the DNA in cell to change its transcription and activity level. JAK-STAT pathway mainly consists of three components: (1) a receptor; (2) Janus kinase (JAK) and (3) a signal transducer and activator of transcription (STAT) protein. The receptor can be activated by interferon, interleukin, growth factor or other chemical messenger, and such activation leads to the phosphorylation of JAK itself. Then, the STAT protein bonds to the phosphorylated receptor, so that STAT is phosphorylated by JAK. After that, the phosphorylated STAT protein is isolated from the receptor, then dimerized and translocated into cell nucleus, thereby bonding to specific DNA site and changing transcription (Scott, M. J., C. J. Godshall et al. (2002). “Jaks, STATs, Cytokines, and Sepsis” Clin Diagn Lab Immunol 9(6): 1153-9).
JAK family plays a role in the cytokine-dependent regulation of proliferation and function of cells involved in immune response. At present, there are four known mammalian JAK family members: JAK1, JAK2, JAK3 and TYK2 (Tyrosine kinase 2). The JAK proteins have a size ranging from 120 kDa to 140 kDa, and comprise 7 conserved JAK homology (JH) domains. One of them is a functional catalytic kinase domain, and another is a pseudokinase domain which effectively exerts a regulatory function and/or acts as a docking site for STATs (Scott, Godshall et al. 2002, supra).
At present, the inhibitors for Janus kinase or relevant kinases have been reported, for example, in WO9965909, US20040198737, WO2004099204, WO2004099205, WO200142246, WO200472063, WO9962908, WO2007070514, etc.